$$F=-kx$$
$\left\{\begin{matrix}k=-\frac{F}{x}\text{, }&x\neq 0\\k\in \mathrm{C}\text{, }&F=0\text{ and }x=0\end{matrix}\right.$
$\left\{\begin{matrix}k=-\frac{F}{x}\text{, }&x\neq 0\\k\in \mathrm{R}\text{, }&F=0\text{ and }x=0\end{matrix}\right.$
$F=-kx$